One of the well known solidification purifying process for semiconductors and metal such as silicon and the like is a solidification purifying process using an electron beam melting method. According to this method, as indicated in non-patent document 1, for example, impurities such as phosphorus (P) are vaporized continuously from the to-be-refined material over a water-cooled copper hearth. Then, this to-be-refined material is dripped onto a water-cooled copper casting mold. Next, while an electron beam is irradiated onto a bath level of the dripped to-be-refined material, this to-be-refined material is solidified in one direction from a bottom side of the water-cooled copper casting mold.
However, according to this method, silicon before being solidified and purified is being newly and continuously added constantly to the melt pool of the to-be-refined material (for example, silicon) within the water-cooled copper hearth. Therefore, inside the melt pool, there is a mixture of silicon, from which impurities such as phosphorus are evaporated, and silicon which includes impurities. As a result, according to this method, the purifying effect of the metal is low compared to an instance in which metal is solidified and purified by solidifying in one direction after melting the entire amount of the metal to be solidified and purified (to-be-refined material)
This is because the above solidification purifying process described in the non-patent document 1 is fundamentally the same as the zone melting purifying process. Generally speaking, after the metal to be solidified and purified is melted in its entirety, it is known that, comparing an instance in which the molten metal is solidified in one direction and another instance in which a part of the metal is melted as in zone melting and a solidification is made by sequentially moving the molten zone, the solidification purifying effective rate in the previous instance is higher than the solidification purifying effective rate in the latter instance.
Furthermore, according to the method described in the non-patent document 1 indicated above, as the height of the freeze layer increases, there is a decrease in the temperature gradient of the liquid phase near the phase boundary (solidification interface) between the liquid phase and the solid phase in a direction perpendicular to this solidification interface. As a result, near this solidification interface, a compositional overcooling phenomenon, described later, is more likely to occur. As a matter of fact, an analysis of the density distribution of impurities inside the ingot obtained by an industry level experimental apparatus indicates that the purification effect drops prominently at a position surpassing approximately 50 to 60 percent of the depth of this ingot.
In order to solve the problems described above, a solidification purifying process is suggested using a mechanism in which the water-cooled copper casting mold is rotated. (See, for instance, Non-Patent Document 1 and Patent Document 1.)
However, this method requires a device equipped with a mechanism which rotates the casting mold and reverses the direction of this rotation at an appropriate time interval. Thus, there is a problem in that the equipment becomes too complicated.
Further, in order to actually enhance the purifying effect, it is necessary to rotate the casting mold at a high speed. In this case, there is a problem in that the molten metal (melt pool) might protrude from the casting mold due to centrifugal force.
When the casting mold is not rotated, the silicon forms a thin solidifying layer, i.e., a scull at the wall surface of the water-cooled copper casting mold. Meanwhile, when the casting mold is rotated at a high speed, this scull disappears. Thus, the molten metal of silicon and the copper casting mold contact each other directly. As a result, it becomes difficult to ignore the influence of contamination of silicon due to copper in the casting mold.
As another solidification purifying process of a metal, a method is disclosed such that, a raw material metal (to-be-refined material) is thrown into a water-cooled crucible, the entire surface of the above raw material metal is irradiated with an electron beam, and is melted in its entirety, and thereafter, deflection coil is controlled to narrow the irradiation range of the electron beam. (See Patent Document 2)
According to this solidification purifying process in which the irradiation range of the electron beam is narrowed, a molten metal portion which no longer receives an irradiation of the electron beam sequentially becomes solidified and becomes a solidified portion. Meanwhile, molten metal is left to an end of one side of the water-cooled crucible. The impurity density of this molten metal part is higher than the impurity density of the solidified portion. Therefore, a purified metal may be obtained by removing this molten metal part and extracting only the solidified portion.
However, according to the solidification purifying process by an electron beam disclosed in Patent Document 2, since the irradiation range of the electron beam is gradually narrowed, there is a problem in that the amount of time required for purification increases because it takes time to move the solidification interface to a lateral direction of the water-cooled crucible (a direction perpendicular to the depth direction). Further, the direction in which the solidification interface moves is perpendicular to the direction in which the electron beam is irradiated. At the same time, the temperature gradient of the liquid phase in the direction in which the solidification interface moves is smaller compared to the temperature gradient of the liquid phase in the direction in which the electron beam is irradiated. Therefore, near this solidification interface, a compositional overcooling is more likely to occur. Therefore, there is a problem in that the purification yield cannot be increased unless the solidification speed is slow enough.
In case of silicon, in particular, the equilibrium distribution coefficient of impurity elements (such as iron (Fe) and aluminum (Al) and the like) excluding boron (B) and P is extremely small. As a result, it is known that these impurities may be removed efficiently by a solidification purification. The equilibrium distribution coefficient is a ratio between the impurity density inside a liquid phase and the impurity density inside a solid phase when the impurity is distributed in a completely uniform manner by a convection flow or a diffusion.
However, in actuality, when the molten metal (liquid phase) is solidified at a finite solidification speed considering productivity, the impurities ejected from the solidification interface into the liquid phase is not timely transported uniformly by diffusion or convection. Instead, the impurities are distributed at the solidification interface at a higher density. The distribution coefficient of the impurities considering such a phenomenon, i.e., the value of the effective distribution coefficient at the solidification interface becomes closer to 1 compared to the value of the equilibrium distribution coefficient. Therefore, the effectiveness of purification declines.
Furthermore, in an actual solidification, the melting point of the liquid phase drops due to the impurity which was pushed to the solidification interface and was thickened. Further, according to a coordination relationship between the melting point of a liquid phase corresponding to the concentration distribution of the impurity and the actual temperature distribution, a non-solidified region appears near the solidification interface once the melting point is exceeded.
Such a phenomenon is called a compositional overcooling. Due to this compositional overcooling, the solidification interface becomes unstable, and loses its flatness. As a result, the solidification interface becomes more bumpy (cell growth), and in more extreme cases, the solidification grows in a dendrite form (an arborized form).
In other words, due to compositional overcooling, the crystal of the silicon grows into the liquid phase in a convex form. As a result, the impurities are pushed to both sides. Therefore, the impurity is segregated in a micro perspective, but is rarely segregated in a macro perspective. Therefore, the solidification purifying effect is lost significantly.
In particular, such compositional overcooling is known to be more likely to occur when the temperature gradient of the liquid phase near the solidification interface is small, when the impurity concentration is high, and when the solidification speed of the liquid phase is high.